1. Field of the Invention
This invention relates to a switching device and, in particular, to a switching device which is used to supply power from a battery selectively to one of plurality loads provided in a car.
2. Background
Conventionally, in a car, in order to supply power selectively to one of a plurality of electrical parts (which are hereinafter referred to as loads) according to the operations of operation switches such as an ignition key, a light switch, an audio switch and the like, there are provided a large number of switching circuits.
In this case, FIG. 8 shows a schematic view of the above-mentioned conventional switching circuits. In particular, a battery 1 is connected to a junction block (J/B) 2, while the junction block 2 is connected to operation switches SW1, SW2, - - - which are respectively disposed on an operation panel 3. In the junction block 2, there are provided switching circuits which correspond in number to the operation switches SW1, SW2, - - - . The respective switching circuits turn on or off the connection between a power line from the battery 1 and wires respectively connected to the respective loads in accordance with the operations of the operation switches SW1, SW2, - - - .
Due to this arrangement, the battery power can be supplied selectively to one of the loads through the junction block 2 in accordance with the operations of the respective operation switches SW1, SW2, - - - . For example, if a headlight switch is moved on, then the power line from the battery 1 is electrically connected to wires respectively leading to headlights 4A and 4B, so that the battery power can be supplied to the headlights 4A and 4B to thereby turn on the headlights 4A and 4B.
Besides the loads such as the headlights 4A and 4B to which the power is directly supplied through the junction block 2, there are also disposed other types of loads, such as motors 5A and 5B respectively used to drive power windows, to which the power output from the junction block 2 is supplied through switching circuits 6A and 6B. These switching circuits 6A and 6B can be controlled or switched by operation switches 7A and 7B, respectively.
The present junction block 2 is actually structured as shown in FIG. 9. That is, the junction block 2 includes a plurality of relays L1, L2, L3, - - - . These relays L1, L2, L3, - - - are divided to relays of a type that the on/off states thereof are controlled directly by their corresponding operation switches SW to thereby apply the current directly to their corresponding loads, such as the relays L1 and L2 which respectively correspond to the above-mentioned headlights 4A and 4B and the on/off states of which are controlled directly by their corresponding operation switches SW1 and SW2 to thereby be able to apply the current directly to their respective loads or the headlights 4A and 4B; and, relays of another type that the on/off states thereof are controlled in accordance with the state of an ignition switch 8, such as the relay L3.
Among the relays, to the relays L1 and L2, there is supplied the battery power from the battery 1 through a fusible link (FL) 9 and fuses F1 and F2. In such power supply, if a large current of an allowable value or more flows through the power line connecting the battery 1 and junction block 2, then the fusible link 9 is blown out, and, if an overcurrent of an allowable value or more flows through a wire (a harness) connecting the junction block 2 and each of the loads, then the fuses F1 and F2 are blown out, thereby being able not only to prevent the whole power line from emitting smoke or catching fire but also to prevent an overcurrent from flowing into the loads. Similarly, to the relay L3, there is supplied the battery power from the battery 1 through the fusible link 9, while the output terminal of the relay L3 is connected to the respective loads 5A and 5B through fuses F3, F4 and relays L4, L5.
By the way, in recent years, with the progress of the semiconductor manufacturing technology, it has been easy to obtain semiconductor switches which are excellent in performance and are inexpensive. In view of this, there has been proposed a switching circuit which uses such semiconductor switches instead of the above-mentioned relays L1, L2, - - - that can be operated by means of mechanical contacts.
This type of switching circuit, generally, has a protection function which can protect a semiconductor switch against an overcurrent or overheating. Therefore, when a current of a rated current or larger flows into the semiconductor switch, or when the semiconductor switch is raised up to a given temperature or higher, the semiconductor switch can be forcibly turned off to thereby protect the semiconductor switch.
Now, in FIG. 10, as an example of a switching circuit using the above-mentioned semiconductor switch, there is shown a switching circuit 30 which is referred to as an intelligent power switch. The switching circuit 30 is substituted for each of the above-mentioned relays L1, L2, - - - , that is, it is connected at the position of each of the relays L1, L2, - - - . For example, referring to a case in which the switching circuit 30 is connected in place of the relay L1, the fuse F1 (FIG. 9) is connected to a power input terminal 12, while the load 4A is connected to an output terminal 13. Also, the operation switch SW1 is connected to a control signal input terminal 14. Here, in the switching circuit 30, in fact, if the operation switch SW1 is moved ON, then, as an on control signal, for example, a control voltage of 5 V! is supplied from the operation switch SW1 to the control signal input terminal 14; and, if the operation switch SW1 is moved OFF, then a control voltage generation part (not shown), which prevents the supply of the control voltage, is interposed between the operation switch SW1 and control signal input terminal 14.
The switching circuit 30 includes an abnormal signal output part 41 which is used to notify an external device of the abnormal condition of the switching circuit 30 in accordance with an output voltage value V.sub.OUT from the semiconductor switch. The abnormal signal output part 41, as shown in FIG. 8, is connected to an abnormal display part 43 through a CPU (central processing unit) 42. In particular, if an overvoltage is applied to the semiconductor switch 32 of the switching circuit 30, or if an overcurrent flows in the semiconductor switch 32, or if the semiconductor switch 32 is overheated, then the protection function of the switching circuit 30 is activated to thereby forcibly move the semiconductor switch 32 off. That is, the abnormal signal output part 41 detects this abnormal condition and sends an abnormal signal to the CPU 42. In accordance with the abnormal signal, the CPU 42 judges which switching circuit 30 is out of order and what kind of abnormal condition has occurred, and allows the abnormal display part 43 to display the judgment results.
Next, description will be given below of the structure of the switching circuit 30 which includes the above-mentioned type of intelligent power switches. The switching circuit 30 applies a power voltage V.sub.B to a .pi. MOS-FET 32 through the input terminal 12 thereof connected to the fusible link FL9 and allows a driver 33 to control the on/off states of the .pi. MOS-FET 32.
Also, the switching circuit 30 further includes an overvoltage detect circuit 34 which, when the power voltage V.sub.B is an overvoltage, detects the overvoltage, an current detect circuit 35 which compares a voltage value based on a current value flowing between the drain and source of the .pi. MOS-FET 32 with a reference voltage V.sub.ref from a reference voltage generation circuit 33A to thereby detect an overcurrent, and a temperature detect circuit 36 which compares a temperature voltage value V.sub.T obtained by a temperature sensor (not shown) disposed in the neighborhood of the .pi. MOS-FET 32 with the reference voltage V.sub.ref to thereby detect the overheating of the .pi. MOS-FET 32. The detect results of the respective detect circuits 34, 35 and 36 are inputed to a NOR circuit 37. Also, to the NOR circuit 37, there is input a control voltage V.sub.IN through an inverter 38.
The output of the NOR circuit 37 is applied to the driver 33 and charge pump 39. The charge pump 39 is operated only when the output of the NOR circuit 37 is positive logic; in particular, the charge pump 39 generates a drive voltage necessary to turn on the .pi. MOS-FET 32 by increasing a power voltage V.sub.DD stabilized by a regulator 40, and supplies the thus generated drive voltage to the driver 33. If the output of the NOR circuit 37 is positive logic, the driver 33 applies the drive voltage generated by the charge pump 39 to the gate of the .pi. MOS-FET 32 to thereby turn on the .pi. MOS-FET 32. On the other hand, if the output of the NOR circuit 37 is positive logic, the driver 33 does not apply the above-mentioned drive voltage to the gate of the .pi. MOS-FET 32 to thereby turn off the .pi. MOS-FET 32.
Also, in the switching circuit 30, the output voltage V.sub.OUT is supplied to the abnormal signal output part 41 through an inverter 44. The abnormal signal output part 41 includes a channel MOS-FET 41A which is turned off when the .pi. MOS-FET 32 is controlled ON and the output voltage V.sub.OUT is a high voltage. On the other hand, when the .pi. MOS-FET 32 is controlled or turned off and the output voltage V.sub.OUT is a low voltage, the channel MOS-FET 41A is turned on. Also, the drain terminal 41B of the channel MOS-FET 41A is pulled up.
Therefore, in the CPU 42 (FIG. 8), when there is found no potential difference between the drain terminal 41B and source terminal 41C of the MOS-FET 41A, it can be judged that the protection function is not in operation in the switching circuit 30 (that is, no abnormal condition is found). On the other hand, if there is found any potential difference between the drain terminal 41B and source terminal 41C of the MOS-FET 41A, it can be judged that the protection function is in operation in the switching circuit 30 (that is, an abnormal condition has occurred).
By the way, in the switching circuit using the above-mentioned semiconductor switches, in order to turn on each of the semiconductor switches, a drive voltage higher than the power voltage must be applied to the control input terminal of the semiconductor switch. For this reason, in the switching circuit of this type, there is provided the above-mentioned charge pump 39 in order to generate such drive voltage.
However, since the charge pump 39 basically includes an oscillator for senerating a square wave pulse and a booster circuit for boosting the power voltage using the square waveform pulse, there occurs oscillation noise which interferes directly with the power line, or there occurs radiant noise which interferes with the signal line. As a result of this, parts connected to the power line easily malfunction and, especially when the parts are digital equipment, they more easily malfunction.
In view of the above, conventionally, there is provided on the power line a noise remove circuit which is used to remove the noise generated from the charge pump 39. However, because the noise remove circuits are provided on the charge pumps 39 one for one, the structure of the switching circuit is complicated in correspondence to the provision of the noise remove circuits.
Also, in the above-mentioned switching circuit 30 including switches which are referred to as intelligent power switches, since the abnormal signal output part 41 is directly connected to the power line, the abnormal signal output part 41 is influenced greatly by the noise, which raises a possibility that the abnormal signal output part 41 can output a wrong abnormal signal due to the noise.